Process for the manufacture of carbon textiles



United States Patent 3,441,378 PROCESS FOR THE MANUFACTURE OF CARBCN TEXTILES Rostislav Didchenko, Cleveland, Ohio, assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed May 10, 1966, Ser. No. 548,862 Int. Cl. Cillh 3. /07; B06111 l/00 US. Cl. 23209.1 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to carbon textiles and more particularly to an improved method for manufacturing the same. As used herein and in the appended claims carbon includes both the non-graphitic and graphitic forms of carbon.

Carbon is an element which possesses many interesting and useful chemical and physical properties. It is a material which both can be found in nature and produced synthetically. Carbon is a readily processible material and can :be fashioned into almost any intricate shape or pattern. Today, the uses of carbon in commerce and industry are myriad.

Presently, most of the carbon articles used in industry are produced by a process which comprises mixing nongraphitic carbon particles with a car-bonizable binder, extruding or molding the so-produced mixture into the desired shape or article and, subsequently, heating it to a temperature sufiicient to carbonize the binder phase. If, during this heating the maximum temperature which the resultant article experiences is of the order of 700-900 C., it is said to be a non-graphitic all carbon article. However, if said article is further heated to a temperature of the order of 2000-2500 C. and higher, it is said to be converted to a graphitic form of carbon and is generally called graphite.

Recently, there has been introduced to the carbon art, carbon in the form of a textile. This form of carbon is unique in that it possesses the flexibility of a textile While at the same time is characterized by the electrical and chemical properties associated with conventionally formed carbon articles.

US. Patent 3,011,981 which issued Dec. 5, 1961 to W. T. Soltes describes and claims a method for manufacturing carbon in a textile form. Briefly, the process disclosed in said patent comprises heating a cellulosic textile in an inert atmosphere at a progressively higher temperature until substantial carbonization of said textile occurs. The resultant product possesses the chemical and physical attributes evidenced by conventionally formed carbon articles while at the same time it retains the flexibility and other physical characteristics associated with the textile starting material, such as hand and drape.

A textile form of fibrous graphite is disclosed and claimed in US. Patent 3,107,152, which issued to C. E. Ford and C. V. Mitchell on Oct. 15, 1963. Broadly stated, the process for producing fibrous graphite disclosed therein comprises heating a cellulosic starting material in an inert atmosphere at progressively higher temperatures for various times until a temperature of about 900 C. is achieved followed by further heating in a suitable protec- "ice tive atmosphere at higher temperatures until substantial graphitization occurs. The product produced by this process exhibits the chemical and physical properties generally associated with conventionally fabricated graphite while, at the same time, it retains the textile characteristics of the starting material.

In all thermal processes for the production of carbon textiles from cellulose, the most critical part of the processing takes place during the low temperature pyrolysis phase of the carbonization treatment, i.e., while it is being heated at temperatures up to about 300 to 400 C. In this temperature range, the mechanism by which cellulose is converted into a carbonaceous char determines not only the amount of undesirable tarry by-products formed, and, therefore, the yield of carbon residue, but it also has a great influence on the physical properties of the final product.

In the past, the first stages in the carbonization of cellulose usually have been conducted either under inert or vacuum conditions. Carbon textiles produced by these techniques possess highly desirable chemical and physical properties, however the yield of carbon obtained is gen erally about 40 percent of theoretical. This low yield is due to the formation of volatile by-products such as carbon dioxide, carbon monoxide, formaldehyde, gloxal and heavy tar fractions which vaporize and remove carbon from the basic cellulosic unit or structure.

Accordingly, it is the principal object of the invention to provide a method for producing carbon textiles from cellulosic textiles in near theoretical yields.

An additional object of the invention is to provide a process for producing carbon textiles which have improved physical properties.

A. still further object of the invention is to provide a process for producing carbon textiles wherein the amount of volatile by-products formed during the pyrolysis of the cellulosic starting material is substantially reduced.

Broadly stated, the objects of the invention are accomplished by a process which comprises heating a cellulosic textile in the temperature range of from ambient temperature to about 400 C. and in the course of said heating subjecting it to a reactive gas containing atmosphere followed by further heating of the resultant chemically modified cellulosic product in an essentially inert atmosphere to a temperature sufiicient to produce a substantially all carbon textile.

In the practice of the invention, cellulose in any textile form may be employed as a starting material, i.e., it may be in the form of felt, cloth, tow, yarn, or the like.

Suitable reactive gas forming materials are volatile acid anhydrides and their derivatives and salts which act as Lewis acids or bases. These gases are introduced into the atmosphere in which the pyrolysis takes place, which may be a partial vacuum, air or an inert gas, such as argon or nitrogen. The reactive gases employed must be strongly hygroscopic and react with cellulose as a dehydrating agent to form cellulosic intermediates which upon subsequent carbonization yield close to theoretical amounts of carbon in a form which still retains the textile characteristic of the starting material, such as hand and drape.

Some specific reactive materials which have been evaluated and found to be readily amenable to the practice of the invention are: phosphorous pentachloride, phosphorous tribromide, ammonium chloride, ammonium bromide, phosphorous pentoxide, phosphorous trichloride, phosphorous oxychloride, sulfur oxychloride and hydrochloric acid.

The temperature at which reactive gaseous materials may effectively be employed ranges from ambient to about 400 C. This is due to the fact that the reactive gas must be brought into contact with the cellulosic textile before it has been substantially thermally converted to carbon.

The reactive gaseous material may be introduced into the atmosphere in which the pyrolysis takes place as a gas directly, with or without a carrier gas, or it may be introduced indirectly, i.e., by vaporizing a volatile liquid or solid. When the latter technique is used, a carrier gas is also usually employed. The partial vapor pressure of the reactive gaseous material in the pyrolysis zone can range from a few to several hundred torr. However, a partial vapor pressure above about 200 torr should be avoided as the structure of the starting textile can be disrupted if the initial reaction between it and the reactive material is allowed to occur too vigorously.

Once the cellulosic textile has been exposed to and reacted with the reactive gaseous atmosphere, the system is then usually purged with an inert gas to remove any remaining reactive material which might still be present in said system. The partially chemically modified cellulosic textile is then subjected to further heating in an inert atmosphere until substantial carbonization occurs, i.e., it is heated to a temperature of about 700-900 C. or higher. This product can then be subjected to further heating to temperatures of the order of 20002500 C. and higher, so as to produce a graphite textile product.

It should be noted that it is often not essential that the reactive gaseous atmosphere be removed from the system while the chemically modified cellulosic intermediate is being heated to carbonizing temperatures. This is due to the fact that at temperatures above about 400 C. the reactive gaseous materials do not react with or attack the resulting essentially all carbon structure. However, when air or an oxygen containing gas is utilized as a carrier, it is necessary to remove it from the carbonizing system at temperatures in excess of about 450 C. as incipient oxidation of carbon occurs at about this temperature.

In addition, it should be mentioned that some reactive compounds may be reduced to their elemental components if they are allowed to remain in the system as the chemically modified cellulosic material is carbonized. Upon cooling, these elemental components might be deposited in or on the resulting carbon textile. Accordingly, if the presence of such components in the end product is undesirable, the reactive materials from which they are produced should be purged from the carbonizing apparatus prior to the carbonization treatment.

In accordance with the teachings of the invention, carbon textiles were produced as described in the following examples:

EXAMPLE I A six by four centimeter rectangular piece of viscose rayon cloth weighing 1.45 grams was wrapped around a quartz mandrel and positioned inside of a cylindrical quartz reaction tube which in turn was horizontally mounted in the center of an electric tube furnace. The quartz reaction tube was provided with ports at its ends for the ingress and egress of gases. A thermocouple was located within the interior of the quartz reaction tube to monitor the temperature of the atmosphere therein. The ingress port was connected to an auxiliary preheater furnace where the reactive gas was generated by heating a boat containing about 2 grams of phosphorous pentachloride. A continuous stream of dry air, as a carrier gas, was introduced into the preheater furnace by a suitable means at a rate of about 250 ml. per minute. This gas was continuously flowed over the boat to pick up the issuing phosphorous pentachloride vapors. The resultant gaseous mixture was then flowed over the cloth sample while both furnaces were heated to 100 C. in minutes. The preheater furnace was held at this temperature in order to continuously produce about 10 mm. partial vapor pressure of phosphorous pentachloride in the system. The temperature of the reaction furnace and, accordingly, the sample therein was uniformly raised to 300 C. in about minutes. At this point, the flow of phosphorous pentachloride was terminated and the air in the system was replaced with nitrogen. The sample was then completely carbonized by heating it in the existing inert atmosphere to a temperature of about 975 C.

The resultant carbonized cloth weighed 0.62 gram (corresponding to a weight yield of approximately 96 percent of theoretical) and had the dimensions of 4.5 x 3.3 cm. (corresponding to a linear shrinkage of about 20 percent).

EXAMPLE II A six by five centimeter piece of rayon cloth weighing 2.00 grams was positioned in an apparatus essentially identical to that described in Example I except that said apparatus was provided with a means for continuously delivering about 30 mm. vapor pressure of phosphorous tribromide to the system. This was accomplished by holding phosphorous tribromide in an in-line trap at 70-75 C. and allowing the resultant vapors to flow into the preheater assembly. The reactive gas was then mixed with dry air and flowed through the system while the preheater and reaction furnaces were simultaneously heated to 300 C. in about one hour. The carbonizing step was then accomplished by heating to 975 C. in a nitrogen atmosphere as described in Example I.

The resultant carbon textile weight 0.80 gram (corresponding to a weight yield of approximately 90.0 percent of theoretical) and had the dimensions of 4.4 by 3.7 cm. (corresponding to a linear shrinkage of about 26 percent).

While the foregoing examples have described only one species of apparatus suitable for the practice of the invention, it will be readily appreciated by those skilled in the art that many other equipment variants may also be employed to realize the benefits afforded by the invention. One of these is described in Example III.

EXAMPLE III A mixture, by weight, of 3 parts of ammonium chloride to one part ammonium bromide was suitably positioned in a conventional carbonizing furnace along with a six by four centimeter piece of rayon cloth weighing 1.75 grams. The amount of salt mixture, 0.5 gram, was completely evaporated as the furnace was heated to a temperature of about 500 C. in 2 hours. The chemically modified rayon cloth sample therein was then carbonized by heating it in said furnace to a temperature of 975 C.

The resultant carbon textile weighed 0.72 gram (corresponding to a weight yield of approximately 92 percent of theoretical) and had the dimensions of 4.2 by 2.8 cm. (corresponding to a linear shrinkage of about 30 percent).

In addition to the specific reactants recited in the foregoing examples, viscose rayon specimens were also beneficially converted to carbon textiles by employing the reactants listed in the following table.

TABLE Yield, in Yield, in percent at percent of Type of sample Reactant Carrier gas 975 C. theoretical Viscose rayon None N 2 16. 0 36. 2 D 19. 5 43. 2 33.0 74. 2 25. 8 57. 9 23. 0 51. 7 27.0 60. 7 H 22. 8 51. 2 NH Cl 39. 3 88.3 Do Pyr HCL... N2 25.0 56.2

5 6 a partial vapor pressure below about 200 torr to form 7. The process of claim 1 wherein said cellulosic textile a chemically modified cellulosic intermediate, and further is y heating said cellulosic intermediate in an inert atmosphere The Process of Claim 1 wherein Said Cellulosic @Xtfle to a temperature in excess of 700 C. to carbonize said 15 Vlscose rayon- 9. The process of claim 1 wherein said carbon textile cellulosic intermediate. 5

ls heated to graphitizmg temperature.

2. The process of claim 1 wherein said reactive material is vaporized phosphorous pentachloride. References Cited 3. The process of claim 1 wherein sa1d reactlve material is vaporized phosphorous tribromide. UNITED STATES PATENTS 4. The process of claim 1 wherein said reactive material 10 3,305,315 2/1967 Bacon et a1. 23-2091 is vaporized ammonium chloride, 3,333,926 8/1967 MOYCI 6t 31 23209.1 5. The process of claim 1 wherein said reactive material 3,337,301 8/1957 Mcwhofter at is a mixture of ammonium chloride and ammomum bro- EDWARD L MEROS Primary Examiner. mide vapors. 5

6. The process of claim 1 wherein said reactive ma- U,S CL X R terial is phosphorous trichloride. 8116; 23--209.4 

